Intergovernmental Panel Climate Change Fourth Assessment Report (2007)

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The World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) established the Intergovernmental Panel on Climate Change (IPCC) in 1988. Membership in the IPCC is open to members of the UN and WMO. The role of the IPCC is to assess, on a comprehensive, objective, open and transparent basis, the scientific, technical and socio-economic information necessary to understand climate change, as well as potential impacts, adaptative and mitigative options.

Structure of the IPCC Report

The Summary for Policy Makers (Fig. 1) for the Fourth Assessment Report was released in February 2007. Representatives from 113 governments reviewed and revised the summary report line-by-line during the course of the week before adopting it and accepting the underlying report. The other IPCC reports are expected later in 2007. The Working Group II report on climate impacts and adaptation will be launched in Brussels on April 6, 2007. The Working Group III report on mitigation will be launched in Bangkok on May 4, 2007. The Synthesis Report will be adopted in Valencia, Spain on November 16, 2007. Together, the four volumes will make up the IPCC’s fourth assessment report. Previous reports were published in 1990, 1995 and 2001. The full underlying report – “Climate Change 2007: The Physical Science Basis” – will be published by Cambridge University Press.

Climate modelling

In the Summary Report, the IPCC concluded that major advances in climate modelling and the collection and analysis of data now give scientists very high confidence (at least a 9 out of 10 chance of being correct) in their understanding of how human activities are causing the world to warm, which is a higher level of confidence than could be achieved during the last assessment report in 2001. The summary report confirms that the marked increase in atmospheric concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) since 1750 is the result of human activities. A greater degree of warming would likely have occurred if emissions of pollution particles and other aerosols had not offset some of the impact of greenhouse gases, mainly by reflecting sunlight back out to space.

Global warming and sea level rise

The Summary Report describes an accelerating transition to a warmer world marked by more extreme temperatures including heat waves, new wind patterns, worsening drought in some regions, heavier precipitation in others, melting glaciers and Arctic ice and rising global average sea levels. For the first time, the report provides evidence that the ice sheets of Antarctica and Greenland are slowly losing mass and contributing to sea level rise.

Observations made in relation to coasts and oceans

The best estimates for sea level rise due to ocean expansion and glacier melt by the end of the century (compared to 1989 – 1999 levels) have narrowed from 28 to 58 cm, versus 9 to 88 cm in the 2001 report. However, larger values of up to 1m by 2100 cannot be ruled out if ice sheets continue to melt as temperature rise. The last time the polar regions were significantly warmer than at present for an extended period (about 125,000 years ago), reductions in polar ice volume caused the sea level to rise by 4 to 6 m. Sea ice is projected to shrink in both the Arctic and Antarctic regions. Large areas of the Arctic Ocean could lose year-round ice cover by the end of the 21st century if human emissions reach the higher end of current estimates. The extent of Arctic sea ice has already shrunk by about 2.7% per decade since 1978, with the summer minimum declining by about 7.4% per decade.

Summary of Synthesis Report, and relevant findings for coasts and oceans

The Synthesis Report is based on the assessment carried out by the three Working Groups of the IPCC and provides an integrated view of climate change as the final part of the IPCC’s Fourth Assessment Report. A Summary for Policy Makers of the Synthesis Report was issued on November 17, 2007, and will be very relevant as the agreed scientific and political text summarizing all of the Fourth Assessment Report, as well as a summary that can more readily be understood by the public and responded to by government decision and policy makers. Those aspects most relevant for coasts and oceans are reiterated below.

As expected, land regions have warmed faster than the oceans. Rising sea level is consistent with warming. Global average sea level has risen since 1961 at an average rate of 1.8 mm/yr and since 1993 at 3.1 [2.4 to 3.8] mm/yr, with contributions from thermal expansion, melting glaciers and ice caps, and the polar ice sheets. It is unclear whether the faster rate for 1993 to 2003 reflects decadal variation or an increase in the longer-term trend.

Observed decreases in snow and ice extent are consistent with warming. Satellite data since 1978 show that annual average Arctic sea ice extent has shrunk by 2.7% per decade, with larger decreases in summer of 7.4 % per decade. Mountain glaciers and snow cover have declined in both hemispheres. From 1900 to 2005, precipitation increased significantly in eastern parts of North and South America, northern Europe and northern and central Asia but declined in the Sahel, the Mediterranean, southern Africa and parts of southern Asia. Globally, the area affected by drought has likely increased since the 1970s.

It is very likely that over the past 50 years: cold days, cold nights and frosts have become less frequent over most land areas, and hot days and hot nights have become more frequent. It is likely that: heat waves have become more frequent over most land areas, the frequency of heavy precipitation events has increased over most areas. Since 1975, the incidence of extreme high sea level has increased worldwide, where extreme high sea level depends on average sea level and on regional weather systems.

There is observational evidence of an increase in intense tropical cyclone activity in the North Atlantic since 1970, with limited evidence of increases elsewhere. There is no clear trend in the annual numbers of tropical cyclones, and it is difficult to ascertain longer term trends in cyclone activity, particularly prior to 1970.

Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years. Observations from all continents and most oceans show that many natural systems are being affected by regional climate changes, particularly temperature increases.

Changes in snow, ice and frozen ground have with high confidence increased the number and size of glacial lakes, increased ground instability in mountain and other permafrost regions, and led to changes in some Arctic and Antarctic ecosystems. There is high confidence that some hydrological systems have also been affected through increased runoff and earlier spring peak discharge in many glacier- and snow-fed rivers, and effects on thermal structure and water quality of warming rivers and lakes.

In terrestrial ecosystems, earlier timing of spring events and poleward and upward shifts in plant and animal ranges are with very high confidence linked to recent warming. In some marine and freshwater systems, shifts in ranges and changes in algal, plankton and fish abundance are with high confidence associated with rising water temperatures, as well as related changes in ice cover, salinity, oxygen levels and circulation.

Of over 29,000 observational data series, from 75 studies, that show significant change in many physical and biological systems, more than 89% are consistent with the direction of change expected as a response to warming. However, there is a notable lack of geographic balance in data and literature on observed changes, with marked scarcity in developing countries.

There is now higher confidence than in the Third Assessment Report in projected patterns of warming and other regional-scale features, including changes in wind patterns, precipitation, and some aspects of extremes and sea ice. Regional-scale changes include:

  • warming greatest over land and at most high northern latitudes and least over Southern Ocean and parts of the North Atlantic Ocean, continuing recent observed trends in contraction of snow cover area, increases in thaw depth over most permafrost regions, and decrease in sea ice extent. In some projections, Arctic late-summer sea ice disappears almost entirely by the latter part of the 21st century;
  • very likely increase in frequency of hot extremes, heat waves, and heavy precipitation;
  • likely increase in tropical cyclone intensity; less confidence in global decrease of tropical cyclone numbers;
  • poleward shift of extra-tropical storm tracks with consequent changes in wind, precipitation, and temperature patterns;
  • very likely precipitation increases in high latitudes and likely decreases in most subtropical land regions, continuing observed recent trends.

There is high confidence that by mid-century, annual river runoff and water availability are projected to increase at high latitudes (and in some tropical wet areas) and decrease in some dry regions in the mid-latitudes and tropics. There is also high confidence that many semi-arid areas (i.e., Mediterranean region, western United States, southern Africa and northeast Brazil) will suffer a decrease in water resources due to climate change.

Projected regional impacts in Europe include a magnification of regional differences in Europe’s natural resources and assets. Negative impacts will include increased risk of inland flash floods, and more frequent coastal flooding and increased erosion (due to storminess and sea level rise). Mountainous areas will face glacier retreat, reduced snow cover and winter tourism, and extensive species losses (in some areas up to 60% under high emissions scenarios by 2080). In southern Europe, climate change is projected to worsen conditions (high temperatures and drought) in a region already vulnerable to climate variability, and to reduce water availability, hydropower potential, summer tourism and, in general, crop productivity. As seen in Greece, Portugal and Spain climate change is also projected to increase the health risks due to heat waves, and the frequency of wildfires.

Arctic impacts, including Scandinavia, are reductions in thickness and extent of glaciers and ice sheets and sea ice, and changes in natural ecosystems with detrimental effects on many organisms including migratory birds, mammals and higher predators. For human communities in the Arctic, impacts particularly those resulting from changing snow and ice conditions are projected to be mixed. Detrimental impacts would include those on infrastructure and traditional indigenous ways of life. In both polar regions, specific ecosystems and habitats are projected to be vulnerable, as climatic barriers to species invasions are lowered.

For small islands, including those in the Atlantic Ocean and Mediterranean Sea, sea level rise is expected to exacerbate flooding, storm surge, erosion and other coastal hazards, thus threatening important infrastructure, settlements and facilities that support the livelihood of island communities. Deterioration in coastal conditions, for example through erosion of beaches and coral bleaching, is expected to affect local resources. By mid-century, climate change is expected to reduce water in many small islands, i.e. in the Caribbean and Pacific, to the point where it becomes insufficient to meet demand during low-rainfall periods. With higher temperatures increased invasion by non-native species is expected to occur, particularly on mid- and high-latitude islands.

Some systems, sectors and regions are likely to be especially affected by climate change. These systems includes particular ecosystems such as:

  • ‘’’terrestrial:’’’ tundra, boreal forest and mountain regions because of sensitivity to warming; Mediterranean-type ecosystems because of reduction in rainfall; and tropical rainforests where precipitation declines,
  • ‘’’coastal:’’’ mangroves and salt marshes, due to multiple stresses,
  • ‘’’marine:’’’ coral reefs due to multiple stresses; the sea ice biome because of sensitivity to warming.

Water resources will be especially affected in some dry regions at mid-latitudes including arid and semi-arid regions, and in the dry tropics, due to changes in rainfall and evapotranspiration, and in areas dependent on snow and ice melt. In turn, this impacts agriculture in low-latitudes, due to reduced water availability. Low-lying coastal systems are especially affected due to threat of sea level rise and increased risk from extreme weather events, as is human health in populations with low adaptive capacity.

Especially affected regions include the Arctic, because of the impacts of high rates of projected warming on natural systems and human communities ;Africa, because of low adaptive capacity and projected climate change impacts ; small islands, where there is high exposure of population and infrastructure to projected climate change impacts ;Asian and African megadeltas, due to large populations and high exposure to sea level rise, storm surges and river flooding ; and within other areas, even those with high incomes, some people (such as the poor, young children, and the elderly) can be particularly at risk, and also some areas and some activities.

Abrupt or irreversible impacts anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change. Partial loss of ice sheets on polar land could imply metres of sea level rise, major changes in coastlines and inundation of low-lying areas, with greatest effects in river deltas and low-lying islands. Such changes are projected to occur over millennial time scales, but more rapid sea level rise on century time scales cannot be excluded.

Climate change is likely to lead to some irreversible impacts. There is medium confidence that approximately 20-30% of species assessed so far are likely to be at increased risk of extinction if increases in global average warming exceed 1.5-2.5°C (relative to 1980-1999). As global average temperature increase exceeds about 3.5°C, model projections suggest significant extinctions (40-70% of species assessed) around the globe.

Based on current model simulations, the meridional overturning circulation (MOC) of the Atlantic Ocean will very likely slow down during the 21st century; nevertheless temperatures over the Atlantic and Europe are projected to increase. This circulation is very unlikely to undergo a large abrupt transition during the 21stcentury. Longer-term changes cannot be assessed with confidence. Impacts of large-scale and persistent changes are likely to include changes in marine ecosystem productivity, fisheries, ocean CO2 uptake, oceanic oxygen concentrations and terrestrial vegetation. Changes in terrestrial and ocean CO2 uptake may feed back on the climate system.

Adaptation and mitigation options in The Synthesis Report differed from the other three reports in that it also discussed a wide array of adaptation and mitigation options. It recognises that a wide array of adaptation options are available, but more extensive adaptation is required to reduce vulnerability to climate change. There are also barriers, limits and costs, which are not fully understood.

The concept of “mitigation potential” was been developed to assess the scale of greenhouse reductions that could be made, relative to emission baselines, for a given level of carbon price (expressed in cost per unit of carbon dioxide equivalent emissions avoided or reduced). Mitigation potential is further differentiated in terms of “market mitigation potential” and “economic mitigation potential”. Market mitigation potential is the mitigation potential based on private costs and private discount rates (reflecting the perspective of private consumers and companies ), which might be expected to occur under forecast market conditions, including policies and measures currently in place, noting that barriers limit actual uptake. Economic mitigation potential is the mitigation potential, which takes into account social costs and benefits and social discount rates (reflecting the perspective of society; social discount rates are lower than those used by private investors ), assuming that market efficiency is improved by policies and measures and barriers are removed. Mitigation potential is estimated using different types of approaches. Bottom-up studies are based on assessment of mitigation options, emphasizing specific technologies and regulations. They are typically sectoral studies taking the macro-economy as unchanged. Top-down studies assess the economy-wide potential of mitigation options. They use globally consistent frameworks and aggregated information about mitigation options and capture macro-economic and market feedbacks.

Societies have a long history of managing the impacts of weather- and climate-related events. However, additional adaptation measures will be required to reduce the adverse impacts of projected climate change and variability, regardless of the scale of mitigation undertaken over the next two to three decades. Vulnerability to climate change can be exacerbated by other stresses. These arise from, for example, current climate hazards, poverty and unequal access to resources, food insecurity, trends in economic globalisation, conflict and incidence of diseases.

Some planned adaptations to climate change are already occurring on a limited basis. Adaptation can reduce vulnerability especially when it is embedded within a broader sectoral. There is high confidence that there are viable adaptation options that can be implemented in some sectors at low cost, or with high benefit-cost ratios. However, comprehensive estimates of global costs and benefits of adaptation are limited.

Adaptive capacity is intimately connected to social and economic development but is unevenly distributed across and within societies. A range of barriers limit both the implementation and effectiveness of adaptation measures. The capacity to adapt is dynamic and is influenced by a society’s productive base including natural and man-made capital assets, social networks and entitlements, human capital and institutions, governance, national income, health and technology. Even societies with high adaptive capacity remain vulnerable to climate change, variability and extremes.

Both bottom-up and top-down studies indicate that there is high agreement and much evidence of substantial economic potential for the mitigation of global greenhouse gas emissions over the coming decades that could offset the projected growth of global emissions or reduce emissions below current levels. While top-down and bottom-up studies are in line at the global level, there are considerable differences at the sectoral level.

No single technology can provide all of the mitigation potential in any sector. The economic mitigation potential, which is generally greater than the market mitigation potential, can only be achieved when adequate policies are in place and barriers removed. Bottom-up studies suggest that mitigation opportunities with net negative costs have the potential to reduce emissions by around 6 GtCO2-eq/yr in 2030, realizing which requires dealing with implementation barriers.

Intergovernmental Panel on Climate Change reports

Numerous reports have been published by the IPCC since it's conception in 1988, including those covers featured below. Information and summaries can be downloaded from the IPCC publications website.


General website - IPCC
History of relationship between IPCC and UNFCCC
Summary for Policy Makers
The main author of this article is Magdalena Muir
Please note that others may also have edited the contents of this article.

Citation: Magdalena Muir (2020): Intergovernmental Panel Climate Change Fourth Assessment Report (2007). Available from [accessed on 25-09-2021]